This invention relates to new and improved detectors for detecting electrostatic discharge (ESD) events occuring in electronic components and electronic assemblies. The invention is particularly directed to detectors which can be positioned directly on or closely adjacent to the miniature electronic components and printed circuit board assemblies presently in use.
The electronics industry began to experience static problems in the 1960s, typically in the form of damage to components resulting from electrostatic discharges. In layman's language, mini lightning bolts occur within such electronic devices due to the build up and discharge of electrostatic potentials, with the lightning bolts or electrostatic discharges causing damage to components, which damage is not readily detected, even in testing of the components.
This problem has become more severe as technology advances have resulted in smaller and more dense semi-conductors which are more suspectible to damage from such electrostatic discharge events. Electronic device susceptabilities are now routinely as low as 100 volts. Human beings can generally feel events no lower than 3500 volts.
A variety of instruments are currently manufactured for the purpose of measuring electrostatic fields in the area around an electronic assembly work space. Instruments of this type provide an alarm or a record of the measured electrostatic field level when a preset threshold is exceeded. Such instruments are described in U.S. Pat. No. 4,785,294 and the references of record in that patent.
Instruments of this nature are large compared to the electronic assemblies to be protected and are used in the work areas to monitor static field levels, but do not provide for an indication of the occurrence of an actual ESD event at a specific device or on a PC board.
The present invention preferably utilizes a non-destructive direct reading display element. A non-destructive direct reading display element is one which can be read at any time to determine whether or not an ESD event has occurred, and if not, remains capable of indicating a subsequent ESD event. This is in contrast to an indirect reading display element which requires some type of change of condition in order to be read, which change prevents the element from subsequently indicating an ESD event. A photographic film is an example of an indirect reading display element, since the film must be developed to determine whether or not an ESD event has been detected. After developing, the film is no longer useable to indicate an ESD event. A liquid crystal display is an example of a non-destructive direct reading display element. The liquid crystal display element can be read at any time without adversely affecting its capability of indicating a subsequent ESD event, if there has not been a prior ESD event.
U.S. Pat. No. 4,838,653 discloses the use of an array of liquid crystal elements for the measurement of the build up of static electrical charge or electric field strength, ie, for field monitoring. In order to achieve the desired sensitivity, the elements are connected in series. Earlier U.S. Pat. Nos. 4,286,210 and 3,627,408 disclose the use of a liquid crystal display element for an ion indicator or meter for detecting high voltage electric fields. These patents are not concerned with the detection and/or recording of actual ESD events.
Products in the ESD monitoring market focus on monitoring electric fields. While knowledge of the electric field level is helpful, experience with field level measurement has demonstrated that control of electric fields in manufacturing environments is nearly impossible, especially over large areas. The detectors detect non threatening fields from clothing, hair, etc. Also, problems are encountered at work benches and work areas where high voltage equipment is operated (CRT monitors, etc).
Field monitors alarm when threatening fields are present. They detect so many threats that tracking the source of each becomes an unbearable task. Some are real threats, some are not. The distinction is really the stored energy involved. This available energy is actually the destructive power behind an ESD source and is a product of the source capacitance and voltage. The stored energy is described by the equation E=1/(2CV.sup.2). No method exists for measurement of the sources capacitance without a direct connection. Therefore, prior to an event, one cannot measure the energy level of a potential ESD event. To further cloud this issue, insulators are less of a threat because they cannot quickly dump their charge to a device, ie, they have no efficient discharge mechanism. Charge must move for a device to be damaged, and during an ESD event, it moves typically in nanoseconds. No method exists for the proximity measurement of discharge resistance.
Detection of ESD events overcomes these difficulties. An event monitor effectively "listens" for electric field transients generated during an ESD event. The distinction between ESD and other transients is their speed. The strength of an electrostatic discharge event is proportional to the level of the measured pulse. An event monitor is not false triggered by slowly changing fields in the area which causes field monitors to alarm.
The device of U.S. Pat. No. 4,838,653 relies on capacitive voltage division to measure the electric field. Consider a model of the equivalent circuit with two capacitors in series between the voltage source and ground. The voltage across each capacitor is inversely proportional to it's capacitance. The capacitor between the source and the internal node is the stray capacitance between the charged object and the plate of the display. The second capacitor models the display capacitance.
The continuous voltage source is divided by the capacitance ratio between the charged element and display square. The theoretical maximum sensitivity of passive LCD materials is 10.sup.5 volts/meter, far too low for use as an ESD sensor.
Another type of device is shown in U.S. Pat. No. 4,825,152. A piece of photographic film is positioned between two electrodes which serve as an antenna for picking up an ESD event and provide a spark gap. An event of sufficient strength will produce a spark at the gap, which spark is recorded in the photographic film. The film is later developed in the usual matter and visually inspected to determine if an ESD event has been detected and recorded. In order to visually determine the occurrence of an ESD event, this secondary film developing process must occur thereby preventing subsequent use of the film if no spark has occurred.
It is an object of the present invention to provide a new and improved detector for detecting very low voltage electrostatic discharge events at a specific electronic component or on an electronic assembly or PC board. Also, it is an object to provide such a detector which can be directly mounted on the component or assembly to be monitored, and in addition, one which can be directly mounted on and connected to the body of a human assembler. This detector will provide an immediate visual indication of such an event and will not require developing.
It is a particular object of the present invention to provide such a detector which is small and inexpensive and therefore practical for use with the very large number of miniature electronic components and assemblies presently being produced. A further object of the invention is to provide such a detector which operates with a direct reading display element, preferably only a single liquid crystal display element. An additional object is to provide such a dectector which can detect the low voltage ESD events without requiring amplification or a power source and provide an indication which is readily distinguished by the unaided human eye.
It is an object of the invention to provide such a detector incorporating a display element which changes indication on exposure to a change in voltage level, and an ESD sensor with circuitry for connecting the ESD sensor to the display element, and in the preferred embodiment, with a mounting arrangement for directly mounting the sensor and display onto the device to be monitored. A further object of the invention is to provide such a detector in which in one configuration, the display element and sensor are in a single component, and in another configuration, they are in separate components. An additional object is to provide such a detector which for some display elements, incorporates amplifier and/or latch circuitry.
These and other objects, advantages, features and results will more fully appear in the course of the following description.